1. Field of the Invention
The present invention relates to an electric heating element for fusing thermoplastic materials, and more particularly, to an electric heating element of the type used in the electric heat fusion of thermoplastic pipe fittings and the like.
2. Description of the Prior Art
Electric heat weldable fittings formed of thermoplastic material have been developed and used heretofore. Such fittings generally include an electric resistance heating coil or element positioned adjacent the inside surfaces of the fitting which are to be welded to one or more other thermoplastic members such as plastic pipe sections. The electric heating element is usually a coil of wire disposed in the thermoplastic material of the fitting connected to electric contacts which are attached to an outside surface of the fitting. In welding the fitting to a complementary thermoplastic member or members such as one or more pipe sections, the fitting is positioned in contact with the other thermoplastic member or members, a source of electric current is connected to the contacts, and electric current is supplied to the heating element of the fitting. The wire making up the heating element imposes a resistance to the current which causes it as well as the thermoplastic member or members positioned adjacent thereto to be heated whereby the thermoplastic materials of the fitting and adjacent member or members are fused together. Examples of coupling types of electric heat fusable thermoplastic fittings are described in U.S. Pat. Nos. 4,349,219 issued Sept. 14, 1982 and 4,147,926, issued Apr. 3, 1979. A saddle tee type of electric heat fusable thermoplastic fitting is described in U.S. Pat. No. 4,455,482 issued June 19, 1984.
An electric heat fusable thermoplastic fitting of the coupling type generally includes a heating element which is comprised of an elongated wire formed in a helical configuration disposed within the internal cylindrical thermoplastic sides of the fitting. The saddle tee type of electrically fusable thermoplastic fitting has heretofore included a relatively expensive and elaborate heating element comprised of a spirally configured resistance wire disposed in a manufactured thermoplastic plate. That is, as disclosed in U.S. Pat. No. 4,455,482 mentioned above, the heating element has heretofore been produced from a flat annular plate formed of thermoplastic material in which a spiral groove is formed. An electric heating wire is disposed within the groove and held therein by melted portions of the plate formed with a heated tool, or other similar means. In producing a prior art saddle tee fitting, the plate containing the heating wire is placed in a thermoplastic injection mold for forming the fitting and bent to conform with and form a part of the arcuate inside surface of the fitting followed by the injection of thermoplastic material into the mold. The term "saddle tee" is used herein to mean both self-tapping tees of the type described in U.S. Pat. No. 4,455,482 and branch tees which are not self-tapping.
Spirally configured wire heating elements of the type described above, in addition to being used in electric heat fusable saddle tees, have been used in other applications for fusing thermoplastic material such as in the repair of tears or holes in thermoplastic material coverings. In such an application, a heating element is placed between the layer of thermoplastic material to be repaired and a second layer of thermoplastic material placed over the area to be repaired. Upon the application of electric current to the heating element, the layers of thermoplastic material are fused together.
By the present invention, an improved spirally configured wire heating element for electrically fusing thermoplastic materials positioned adjacent thereto is provided. A method of producing the electric heating element and an improved thermoplastic fitting including the heating element are also provided.